As conventional multi-blade fans of this kind, one including an orifice having a bellmouth-shaped inlet is known and disclosed in, for example, patent document 1. Hereinafter, the multi-blade fan is described with reference to FIGS. 16 and 17.
As shown in the drawings, multi-blade fan 101 includes main plate 102 having opening 103. Upper blade 105 at the side of lateral plate 104 of main plate 102 and lower blade 106 on the opposite side of lateral plate 104 of main plate 102 have different sectional shapes from each other.
In the above-mentioned configuration, when multi-blade fan 101 is rotated, sucked air passes through sucking hole 108 of orifice 107, and is subjected to increasing pressure by lower blades 106 at a low pressure of high air volume and is subjected to increasing pressure by upper blades 105 at a high pressure of low air volume. At this time, one or both of an inlet angle and an outlet angle are different between lower blade 106 and upper blade 105. Thus, a high-performance multi-blade fan can be obtained.
When such a conventional multi-blade fan controls an air volume to be constant by using a property of a fan, it generally detects the number of rotation, a voltage and an electric current depending upon the number of rotation, and the like, at a low pressure of high air volume and at a high pressure of low air volume. However, it is not suitable for controlling of an air volume to be constant because the difference between the number of rotation at a low pressure of high air volume and the number of rotation at a high pressure of low air volume is not large. Furthermore, the fan efficiency is not good. Increase in the difference in the number of rotation and improvement of the fan efficiency have been demanded.    [Patent Document 1] Japanese Patent No. 3507758